The invention relates to fiberoptic connectors, and more particularly to methods and apparatus for reliably testing multi-fiber fiberoptic connectors to ensure that all fiber ends are properly aligned with, and in physical end-to-end contact with, corresponding fiber ends in a mating fiberoptic connector.
The MT (Mechanically Transferable) connector shown in FIG. 1 and MPO (Multi-path Push On) connector shown in FIGS. 2A and 2B originally developed by NTT in Japan, have been deployed primarily in Japan for several years. The advantageous technical features and price/performance ratios of MT and MPO connectors have meant that this non-traditional style of fiberoptic connector is gradually becoming more widely accepted, and is becoming quite widely used in both the U.S. and other worldwide markets. The main advantages of MT and MPO connectors are high optical fiber density (typically 2-12 fibers), small physical size, and low cost. A variant of the MT connector is the MT-RJ connector, which has a smaller design that fits within the footprint of a standard 8 pin modular telephone jack, and is being considered as one of the main contenders by the standards organizations for fiberoptic premise network wiring.
This type of fiberoptic connector has extremely critical dimensional tolerances that must be maintained to ensure acceptable performance and "intermatability" of connectors. As these connectors (the MT, MPO, and MT-RJ) become more widely used in single-mode applications, their geometric tolerances are expected to become even tighter. As fiberoptic cable bandwidth requirements increase, the fiberoptic connectors can become one of the most critical components affecting the overall system performance of a fiberoptic transmission system.
An optical fiber 1 shown in FIG. 4 typically is constructed in three distinct concentric layers, including a 250 micron diameter acrylic jacket 2 which coats the outside of the glass optical fiber. The jacket's main function is to provide basic environmental protection to the glass optical fiber. Without this jacket, just brushing the fiber over another surface could score the glass, leading to a crack which eventually would propagate through the glass, fracturing the fiber and rendering it inoperable. Since the acrylic coating can be colored, it also provides a useful method of fiber identification. The second layer is a 125 micron diameter cladding 3. This has become the standard outer diameter for all but the most unusual and application specific fiber designs. The purpose of the cladding is to contain the light within the 8 micron fiber core 4, using the principal known as "total internal reflection". The secondary purpose of the cladding 3 is to increase the fiber diameter to a level that provides it sufficient mechanical strength, can be fairly easily seen and can be manipulated by human hands. The core is the part of the fiber that carries the light. The core 4 and the cladding 3 constitute one contiguous piece of glass; however, they have different refractive indexes to keep the light within the core. Multi-mode fibers have a typical core diameter of 62.5 microns, as opposed to the 8 micron core diameter typically used in single-mode fibers.
Although the core of a single-mode fiber is much smaller than that of a multi-mode fiber, allowing only a single "mode" to propagate from the input to the output of the fiber dramatically increases the amount of data or "bandwidth" offered by single-mode fibers, when compared to multi-mode fibers. With the rapidly increasing demand for voice, video, and Internet communications, bandwidth can be a scarce and valuable resource. As such, most new long distance fiber deployment is single-mode. Even when using sophisticated multiplexing techniques, the maximum bandwidth capacity of a single fiber may be used up, and there is no other option than to add additional fibers to increase communication capacity. As the number of single fibers being added to a bundle increases, so does the diameter of the cable necessary to contain and protect them. Not only is this expensive, but it can also create problems in already crowded ducts and passages used to route cables. Therefore, manufacturers are looking for ways to achieve smaller physical size, higher performance, more manageable, and less expensive systems, and have begun to manufacture "ribbon fibers". A ribbon fiber as shown in FIG. 5 includes a number of optical fibers (typically 2-12) laid side by side and sleeved with an additional outer coating. This technique provides very high fiber densities, while having the added advantage that installation and maintenance workers are able to handle up to 12 or more fibers at one time.
Fiber preparation can be a very labor intensive and expensive part of terminating or joining fibers together. Having the ability to work on multiple fibers at one time using specialized tools has led to dramatic time and cost savings in optical fiber installation and maintenance for ribbon fiberoptic cable users.
Optical fiber multiplexing and transmitter and receiver technology have made such great technological advances that data transfer rates of the order of Terabytes per second over a "perfect" optical fiber link have been demonstrated, using a combination of various multiplexing and data compression technologies. As a result, engineers and scientists now face the difficult task of simplifying system implementation (without losing performance) to a level such that workers with little experience, crawling down through manhole covers in harsh environments, can be reasonably expected to install and maintain such ribbon fiber links with a high degree of success and reliability.
One of the most important and frequently overlooked factors involved when installing a fiberoptic transmission system is the proper installation and use of fiberoptic connectors. When it is necessary to join or patch two ribbon cables together, there are two primary choices: fusion of optical fibers and use of optical fiber connectors. Fusion involves accurately cleaving all of the fibers to the same length across the ribbon on the two cables to be joined, and then using a specialized machine known as a ribbon fusion splicer, which brings all of the fiber pairs together very accurately along the X,Y and Z axes. An electrical arc applied with a small compressive force pressing the cleaved surfaces together then is used to physically fuse the individual fiber pairs together as one contiguous fiber. This process, when performed properly, and after the application of additional splice protection, can join two multi-fiber ribbon cables together almost as effectively as if they were manufactured as a contiguous piece of fiber. For permanent joints, fusion splicing provides the most economical and robust solution to joining optical fibers.
On the other hand, there are many situations where a permanent joint is not desired, not required, or not feasible, in which case connectors become the only viable alternative technique for joining the fibers. Examples of such applications would include (1) patch panels where reconfiguring of fiber routes may be necessary, and (2) attachment to system or test equipment and applications such as high speed optical back-planes which require automatic connection and disconnection of the optical path as circuit boards are inserted and removed. At the user level "consumers" expect multi-fiber fiberoptic connectors to work in the same way as electrical connectors, that is, the fiberoptic connectors are simply "plugged in" for a pair of MPO or a pair of MT connectors, and everything works. In reality, a great deal of sophisticated technology and precision engineering has to occur to make this happen.
It is important to recognize that the performance of optical connectors can have a dramatic impact on the overall performance, integrity and reliability of an entire optical link. The main "enemies" of an optical signal at a connectorization point are "loss" and "back-reflection". Since the core diameter of a single-mode fiber is only 8 microns, when connecting two fibers together using a connector, a lateral misalignment smaller than 1 micron can cause significant optical power loss at the connector interface. This uses some of the loss "budget", and therefore reduces the distance through which the light can continue to propagate before regeneration or optical amplification is required to maintain the data integrity of the signal. One cause of back-reflection occurs in a connector when two mating fiber ends do not achieve physical contact with each other, creating a small air gap in the transmission path of the signal, and causing "back-reflections" of the laser light from the unmatched interface. Such spurious reflections can affect the stability of the transmission equipment and greatly degrade the useable bandwidth of the fiberoptic transmission system.
Both high loss and high back-reflection are common problems associated with poorly terminated connectors. Loss is most often caused by either a defective ferrule not maintaining stringent lateral tolerances for fiber alignment, a lack of physical contact between two fiber end faces (due to fiber recess or poor endface geometry), or surface imperfections/contamination on one or both of the fiber endfaces. Back reflection most often is caused by fiber recesses in the connector endface or poor endface geometry wherein the connector's ferrules physically come into contact with each other before the mating fiber ends can physically contact each other. This results in back-reflection caused as the laser wave-front hits the silica/air interface of the discontinuous fiber path. Additionally, high fiber endface surface roughness can also increase both back-reflection and loss.
With conventional single fiber connectors, it is relatively easy using modern processes to assure the necessary physical contact between two inter-mating fibers, simply by spherically polishing the ends of each ferrule containing the fiber. By controlling factors such as the radius of curvature, fiber height and apex offset of the polish with respect to the center of the fiber, physical contact between the fibers can be assured, thereby minimizing loss and back reflection. For further explanation, see my U.S. Pat. No. 5,459,564 entitled "Apparatus and Method for Inspecting End Faces of Optical Fibers and Optical Fiber Connectors", issued Oct. 17, 1995, incorporated herein by reference.
For MT and MPO connectors as shown in FIGS. 1 and 2, the problem of maintaining physical contact simultaneously between all 2-12 fibers becomes significantly more difficult to achieve consistently than for a single fiber connector. Since a truly spherical polish can only have one apex, and can thus only resolve the physical contact problem for one fiber, manufactures have had to resort to alternative methods to solve the problem. A common method is to use a polishing process that leaves the fibers very slightly protruding above the ferrules "flat" or "angled flat" endface surface for MT and MPO connectors, respectively.
It should be appreciated that there is a very small difference between leaving enough protruding fiber to ensure physical contact of all fibers, and the alternatives which would be either (1) to leave too much protruding fiber, thereby damaging the fragile fibers when two connectors are mated, or (2) to leave the fibers recessed below the ferrules surface, thereby eliminating any possibility of physical contact.
Another popular polishing technique that has evolved involves polishing the rectangular upper endface 35A of the MT connector 35 (or MPO connector 350) much like the bowed or elongated convex shape found on the top of a long loaf of bread, e.g. as indicated in FIG. 8. The idea is that the centerline 10 of the transverse axis of the connector which contains the row of fiber ends 1 would be slightly higher than the surrounding perimeter of endface 35A, thereby encouraging physical contact. It should be noted that the difference between adequate physical contact of the fiber ends and none at all is only a few microns. Therefore, having an accurate measurement of the surface topography of the connector endface, including the fiber ends, is essential to predicting the performance of all types of multi-fiber connectors.
MT and MPO fiberoptic connectors have a significantly larger endface area of interest than single-fiber connectors do, not only because they contain more fibers, but also because the polish of the connector endface more closely resembles a flat or angled flat surface than a sphere. Thus, the first point of contact between the endfaces of the two ferrules encapsulating the fibers could fall anywhere across a relatively large surface. This first point of contact (with respect to the fiber heights) determines the magnitude of any possible separation or gap between the ends of the corresponding fibers of the mated connectors, and ultimately determines the overall performance of the connector. Until the present invention, there has been no way to determine the location or height of the above-mentioned first point of contact.
To further explain the foregoing difficulty in determining the first point of contact, FIGS. 10A and 10B show a precisely coupled pair of MT connectors 35 and 35-1. All of the corresponding fiber ends extending from the endfaces of connectors 35 and 35-1 are precisely aligned and in perfect end-to-end physical contact so that there is minimal light loss or back-reflection. In contrast, FIGS. 11A-D show some of the various forms of defective end-to-end connector couplings that can occur as a result of imperfect connector endface profiles. FIG. 11A shows a connection where the endface 35A of lower connector 35 has excessive fiber "undercut", preventing any of the ends of any of the corresponding fibers 1 and 1--1 from achieving physical contact. FIG. 11B shows a connection where the endface 35A of lower connector 35 is at an excessive angle relative to a plane perpendicular to the longitudinal axis of guide pin holes 40, whereby the first point of ferrule endface contact prevents the ends of some of the corresponding fibers 1 and 1--1 from achieving physical contact, resulting in both loss and back-reflection. FIG. 11C shows a connection where lower connector 35 has excessive protrusion of fibers 1 above endface 35A, causing the fibers 1 to bend as shown as a result of end abutment forces. Such endfaces result in stress, bending, poor coupling and potential fiber damage. FIG. 11D shows a connection where the radius of curvature of endface 35A of lower connector 35 is too small; the result is that the outermost pairs of corresponding fibers 1 and 1--1 fail to achieve physical contact, which causes harmful loss and back-reflections in those fibers.
To determine the presence of the conditions shown in FIGS. 11A-D, it is necessary to obtain the surface topography profiles of the connector endfaces 35A and 35A-1 and the fiber ends 1 and 1-1 protruding therefrom, using interferometric measurements. Despite the many advantages of the MT and MPO connectors, their design inherently creates many difficulties for interferometric measurement. A first difficulty is that the region of interest on an MT or an MPO connector can be as large as 6.4.times.2.5 millimeters, in contrast to a diameter of only approximately 250 microns (i.e., 0.25 millimeters) being required for a standard single fiber connector. Simply reducing the magnification of the interferometer to view the entire surface is not acceptable, as too much resolution is lost by doing this. Consequently, most manufacturers have independently chosen a "trade-off" magnification, trying to optimize the trade-off between field of view and resolution. Consequently, using the prior art techniques, multiple interferometric measurements of the separate continuous areas of endface of an MT or MPO connector must be made and "pasted" together to build up the required view of the endface surface topography.
A second difficulty is that the material chosen by manufacturers for MT and MPO connectors primarily has been a black epoxy, filled with silica particles. When polished, the resulting endface often looks like "silica islands in a sea of epoxy", as indicated by numeral 9 in FIG. 14, which shows the endface of a typical MT or MPO connector. Good interferometric data representing the surface profile of the endface of an MT or MPO connector often is available only from the highly reflective silica islands. Therefore, it is quite a complex operation to paste all of the captured regions together in three dimensions. Since the good data is situated on such islands 9 as shown in FIG. 12, it becomes necessary to perform broad-band interferometry on MT and MPO connectors, because otherwise it is impossible to know the height relationship of the good data regions with respect to each other. Unfortunately, broad-band interferometry generally is much slower than narrow-band interferometry.
A third difficulty can be understood by comparison to the calibration of conventional single fiber connectors, wherein the precision cylindrical ferrule can be rotated in the interferometer fixture. By watching the interferogram, or alternatively by measuring the apex offset at different rotational orientations, it is possible to verify and compensate for any misalignment between the connector fixture and the interferometers optical axis. Since single fiber connectors use a precision split sleeve to couple two connectors, the endface geometry measurement can be considered to be calibrated because the same surfaces are referenced to measure the endface geometry in the interferometer as are used to locate the ferrule in an actual mating of a pair of joined connectors. In contrast, this is not the case for MT and MPO connectors wherein two precision "guide pins" slide into precision aligned mating holes 40 in a pair of MT or MPO connectors being joined instead of using locating features on the outsides of the ferrules to join a pair of the connectors together. Because the mating reference surface of an MT or MPO connector is not on the outside of the ferrule, and more importantly, because the ferrule cannot be rotated (since it is rectangular), it has not been possible to achieve an accurate calibration of the surface to be measured by the interferometer to obtain a calibrated profile of the connector endface and fiber ends which need to be aligned with and brought into physical contact with ends of corresponding fibers of a mating MT or MPO connector.
As stated earlier, the first point of contact determines the overall performance of a fiberoptic connector. Without an accurate calibration of the measured connector endface data this first point of contact cannot be determined, making any surface topography test data far less useful in judging whether the multi-fiber fiberoptic connector should be accepted, re-worked, or discarded.
Thus, there is an unmet need for a technique and apparatus for providing a profile of a multi-fiber fiberoptic connector endface which is precisely calibrated with respect to a feature of the connector that aligns it to mate with a like multi-fiber fiberoptic connector.